Behind the scenes

Training Grounds Tower in the model of Hogwarts.

The Training Grounds Tower changes in every film adaptation, but the basic shape remains. In Harry Potter and the Philosopher's Stone, it starts off as a tower with lots of turrets. In the second film, the basic shape remains (if drawn out as a floor plan, the original, which was based upon Alnwick Castle, and the new one are the same) but it changes to look more like the Clock Tower until Harry Potter and the Half-Blood Prince, where the tower was changed to another Bell Tower, but was slightly shorter than the other two. However, in some scenes of the castle (notably the scene in which Death Eaters attempt to get past the protection) the old tower is there, as well as the DADA Tower. However, the reason why the filmmakers changed the tower is because when the production team built the Transfiguration Courtyard set, they built the tower like one of the Bell Towers because it saved time and money. This does not explain why it reverts back to its older incarnation midway through the film and then reverts back to its new incarnation towards the end of the film.